This type of pain can be due to the bike fit. If that does not appear to be the answer, suggest that you find a cycling sports physio and ask him/her. You could ask SportsMed if they can recommend a suitable physio.
i'd suspect that recently you've been pushing some bigger gears than you're used to, or going up some steeper hills than usual. Basically, you've pushed yourself a little too much, too soon.
back it off a bit, spin in some lower gears when you are on the bike, ice the knee and maybe take some anti inflammatory medication (eg nurofen)
I had a similar thing early in summer but not quite as painful by the sounds!- I think it was the above - not spinning enough.
Took it easier and hasn't flared up again.
Im surprised you didnt tell David to BTFU...lol
I'm surprised too!
I've told myself to!
On a serious note though David, it sounds awfully like incorrect bike fit to me. I have suffered from similar symptoms during and after riding and after a professional bike fit, it is amazing how pain like that in knees no longer exists.
Noel at the bike station does a professional fit for a small sum of money or free if you buy a new bike from him. Takes about half hour to an hour to get all your measurements right, but man is it worth it. Your comfort levels improve and you just enjoy your riding far more.
I was fitted at Turtur's they do a pretty good job - I've probably done about 3,000km since I bought it, without issue.